Detailed observations of NGC 4151 with IUE – III. Variability of the strong emission lines from 1978 February to 1980 May

• Published in: Monthly notices of the Royal Astronomical Society 1984-01, Vol.206 (1), p.221-237
• Main Authors: Ulrich, M. H., Boksenberg, A., Bromage, G. E., Clavel, J., Elvius, A., Penston, M. V., Perola, G. C., Pettini, M., Snijders, M. A. J., Tanzi, E. G., Tarenghi, M.
• Format: Article
• Language: English
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; Quasars. Active or peculiar galaxies, objects, and systems; Stellar systems. Galactic and extragalactic objects and systems. The universe
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/206.1.221

Observations of the variability of the three strong ultraviolet emission lines in the Seyfert galaxy NGC 4151 (C IV, C III] and Mg II) are used to study the structure of the broad line region and the nuclear energy source of this active galaxy. In contrast to the common situation in QSOs, these emission lines usually are of different widths, indicating stratification in the broad line region. However, at epochs when the power-law component of the continuum is low, all three lines are relatively narrow (half width at zero intensity ∼ 4000 km s−1). The C IV intensity shows the tightest correlation with the continuum, although there is a hysteresis associated with a lag in the response of ∼ 13 days, probably caused by light travel time in the C IV emitting region. The looser correlation of the Mg II emission is consistent with the region emitting this line being larger, perhaps 60 light days in diameter. The lack of significant variations in C III] suggests dimensions exceeding one light year. Data on the line profile variations further permit the kinematic mapping of the broad line region through the response of the line profiles to the sudden continuum increase during 1979 May. Results for the C IV region suggest that the dominant motion is chaotic although there may be a radially outflowing component. From the widths and variability time-scales of the three emission lines we deduce values of the quantity υ2r, which is proportional to a mass. The three independent estimates converge on 0.5–108 M⊙. We show that the behaviour is compatible with the black hole/accretion disc model of Lynden-Bell (1969), since the dimensions of an accretion disc around a black hole of this mass are consistent with the thermal emission at T ∼ 30 000 K which we observe as a short wavelength excess to the power-law continuum in the ultraviolet. The variability data suggest a chain of cause and effect: events are first seen in the thermal component, later in the power-law continuum and last in the emission lines. Thus the power-law continuum fades in response to a decrease in the fuel supply from the accretion disc.